The first super geomagnetic storm of solar cycle 24: "The St. Patrick's day event (17 March 2015)"

Abstract

The first super geomagnetic storm (Dst < -200 nT) of solar cycle 24 occurred on "St. Patrick's day" (17 March 2015). Notably, it was a two-step storm. The source of the storm can be traced back to the solar event on 15 March 2015. At ~2:10 UT on that day, SOHO/LASCO C3 recorded a partial halo coronal mass ejection (CME), which was associated with a C9.1/1F flare (S22W25) and a series of type II/IV radio bursts. The initial propagation speed of this CME is estimated to be ~668 km/s. An interplanetary (IP) shock, likely driven by a magnetic cloud (MC), arrived at the Wind spacecraft at 03:59 UT on 17 March and caused a sudden storm commencement. The storm intensified during the Earth's crossing of the ICME/shock sheath and then recovered slightly after the interplanetary magnetic field (IMF) turned northward. The IMF started turning southward again due to a large MC field itself, which caused the second storm intensification, reaching a minimum value (Dst = -223 nT). It is found that the first step is caused by a southward IMF component in the sheath (between the upstream shock and the front of the MC), whereas the second step is associated with the passage of the MC. The CME that erupted on 15 March is the sole solar source of the MC. We also discuss the CME/storm event with detailed data from observations (Wind and SOHO) and our algorithm for predicting the intensity of a geomagnetic storm (Dst_min) from known IP parameter values. We found that choosing the correct Dst_min estimating formula for predicting the intensity of MC-associated geomagnetic storms is crucial for space weather predictions.